This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the DirecTV satellite system, each “satellite” is actually a constellation of two closely spaced satellites. Each of these satellites comprise a number of transmitters, called transponders, each transponder of which transmits signals on a discrete RF frequency to earth-bound receivers. Each transponder's signal may be modulated with an audio/video program or with other data information. In order to optimize bandwidth utilization, the transponders on one satellite of a constellation transmit signals with right-hand circular polarization while the transponders of the other satellite of the constellation transmit signals with left-hand circular polarization. Satellite receiving systems contain one or more receiving antennae, each of which is switchable to receive either right-hand or left-hand polarized signals. These antennae are each associated with a low noise block (LNB) to amplify the signals received from a chosen set of transponders. The spectrum of RF signals received by each LNB is, in turn, processed by the receiver itself, where a specific frequency associated with a desired program channel is tuned and decoded.
Satellite receiver systems typically remotely power low noise blocks (“LNB”) and/or RF switching devices by supplying a DC voltage. A 600 millivolt peak-to-peak, 22 kHz tone may be superimposed on the DC voltage to enable the receiver to communicate with such devices or to select various satellite configurations. For example, the presence or absence of the tone may select a particular LNB in a multi-LNB system, and the value of DC voltage supplied to the LNB may select a particular polarization.
Satellite receiver systems having LNB power supplies with the ability to superimpose a 22 kHz tone on the output are often configured with a linear pass element. The linear pass element typically has an emitter follower circuit that produces an undesired waveform when connected to a highly capacitive load. The resulting circuit acts like a peak detector causing the output voltage to stay at a value close to the maximum peak level. Essentially, such emitter follower circuits do not have the ability to remove charge from the external capacitance without the aid of some current sinking device.
Historically, satellite receiver systems specified a 30 ma minimum current on LNB/Switching devices and a maximum of 750 nF of capacitance on the output of the supply. Conventional designs have supplemented the minimum 30 ma current with a simple resistor in the satellite receiver power supply to provide enough current sink and produce a proper 22 kHz waveform shape. Satellite receiver systems are now moving toward a specification of zero load current at the LNB/Switch. This low load does not supply enough sinking current to produce a proper 22 kHz wave shape with the 750 nF load. Therefore, a method and apparatus that preserves a proper waveform in newer systems is desirable.